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Friday, July 13, 2018

This is totally unexpected and is a great argument for the evolutionary pressure argument as the prime driver of speciation.

A tropical reef provides a rich nurturing environment with scant evolutionary pressure. Thus we need to expect a lower level of speciation even if we have hundreds of species about. In the cold, it is always harsh and any arrival species will have to change.

ANN ARBOR--Tropical oceans teem with the dazzle and flash of
colorful reef fishes and contain far more species than the cold ocean
waters found at high latitudes. This well-known "latitudinal diversity
gradient" is one of the most famous patterns in biology, and scientists
have puzzled over its causes for more than 200 years.

One frequently advanced explanation is that warm reef environments
serve as evolutionary hot spots for species formation. But a new study
that analyzed the evolutionary relationships between more than 30,000
fish species concludes that the fastest rates of species formation have
occurred at the highest latitudes and in the coldest ocean waters.

Over the past several million years, cool-water and polar ocean
fishes formed new species twice as fast as the average species of
tropical fish, according to the new study, which is scheduled for
publication July 4 in the journal Nature.

"These findings are both surprising and paradoxical," said
University of Michigan evolutionary biologist Daniel Rabosky, lead
author of the study. "A number of hypotheses explain extreme tropical
diversity as the result of faster rates of species formation, but it's
never been tested in fishes.

"Our results are counterintuitive and unexpected, because we find
that speciation is actually fastest in the geographic regions with the
lowest species richness."

The authors admit they cannot fully explain their results, which are
incompatible with the idea that the tropics serve as an evolutionary
cradle for marine fish diversity. The findings also raise questions
about whether the rapid cold-ocean speciation the team documented
reflects a recent and ongoing expansion of marine diversity there.

Common sense suggests that a high rate of new species formation will
eventually lead to impressive levels of biodiversity. But that depends
on how many of the newly formed species survive and how many go extinct.
And extinction rates could not be addressed through the methods used in
the current study.

"The number of species you find in a region is largely a balance
between the rate at which new species form and the rate at which
extinction eliminates them," Rabosky said. "The rapid speciation of
fishes in cold, high-latitude oceans that we documented will only cause
diversity to increase if it is generally higher than extinction.

"Extinction is the missing piece of this puzzle, but it's the most
difficult thing to understand. We're now using both fossils and new
statistical tools to try to get a handle on what extinction might have
been doing in both the polar regions and the tropics."

In the study, Rabosky and colleagues from eight institutions tested
the widely held assumption that species-formation rates are fastest in
the tropics by examining the relationship between latitude, species
richness and the rate of new species formation among marine fishes. They
assembled a time-calibrated evolutionary tree of all 31,526 ray-finned
fish species, then focused their analysis on marine species worldwide.

Genetic data were available for more than one-third of the fish
species analyzed in the study, and the evolutionary tree was
time-calibrated using a database of 139 fossil taxa.

An evolutionary tree, also known as a phylogenetic tree, is a
branching diagram showing the inferred evolutionary relationships among
various species. The tree assembled for this project is one of the
largest time-calibrated phylogenetic trees ever created for any group of
animals, according to Rabosky.

The researchers estimated geographic ranges for most of the marine
fish species, including all species with genetic data. Then they used
complex mathematical and statistical models to estimate the rates at
which different groups of fishes split into new species.

"The computational challenges for analyzing these types of data are
pretty extreme," said study co-author Michael Alfaro, an evolutionary
biologist at the University of California, Los Angeles. The analyses in
the study required the equivalent of thousands of desktop computers
running continuously for many months, he said.

Some of the fastest rates of new species formation occurred in
Antarctic icefish and their relatives. Other temperate and polar groups
with exceptionally high speciation rates include snailfish, eelpouts and
rockfish.

Three of the largest coral reef-associated fish groups--wrasses,
damselfish and gobies--showed low to moderate rates of species
formation.

"The fact that coral reefs support many more fish species than polar
regions despite these lower rates may have a lot to do with their long
history of connectivity and ability to act as a refugia," said co-author
Peter Cowman of the Australia Research Council Centre of Excellence for
Coral Reef Studies, and previously of Yale University. "Our research
certainly paints coral reef diversity in a new light."

"Who would have thought that you'd have these really explosive rates
of species formation happening in the coldest Antarctic waters, where
water is literally at the freezing point and fish like the icefish have
to have all kinds of really crazy adaptations to live there, like
special antifreeze proteins in their blood to keep it from freezing,"
Rabosky said.

###

Rabosky
is an associate professor in the U-M Department of Ecology and
Evolutionary Biology and an associate curator at the U-M Museum of
Zoology.

The authors of the Nature paper, in addition to Rabosky,
Alfaro and Cowman, are U-M's Jonathan Chang, Pascal Title and Matt
Friedman; Lauren Sallan of the University of Pennsylvania; Kristin
Kaschner of the University of Freiburg; Cristina Garilao of GEOMAR
Helmholtz Centre for Ocean Research; Thomas Near of Yale University; and
Marta Coll of the Institute of Marine Science in Barcelona, Spain.

The work was supported in part by grants from the National Science Foundation and by the David and Lucile Packard Foundation.

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